Lithium ion batteries have ubiquitously existed in our daily life, and majority of these batteries are made of cobalt-based electrodes. As the cobalt-based lithium ion batteries get bigger they have thermal runaway problems, which prevent them from the applications to need large battery systems, such as electric vehicles (EV) or large energy storage systems. People have been searching for alternative electrodes which can be used to make large scale lithium ion batteries. Lithium iron phosphate with olivine structure (triphylite) has been identified as one of the most promising cathode materials for large lithium ion batteries since the report of Goodenough's group [1,2] and subsequent studies [2-5], owing to its excellent thermal stability.
The olivine-structured orthophosphate LiFePO4 has an orthorhombic lattice with the space group Pnma and its unit cell parameters are a=10.3290 Å, b=6.0065 Å, and c=4.6908 Å [4]. LiFePO4 can be reversibly delithiated to FePO4. The unit cell parameters for FePO4 phase are a=9.8142 Å, b=5.7932 Å, and c=4.7820 Å [4]. The volume change of unit cells between these two phases is around 6.58%, which is not a big concern for the battery manufacturing. The high stability of LiFePO4 and the minimal changes in the unit cell parameters during the LiFePO4/FePO4 phase transition contribute a good cycle life of the resulting lithium ion batteries. The theoretical capacity of LiFePO4 cathodes is 170 mAh/g with a flat 3.45V charge-discharge potential vs Li/Li0 owing to the Fe3+/Fe2+ redox couple. All the materials sources to form LiFePO4 are abundant, non-toxic, and environmentally friendly. Overall, LiFePO4-based lithium ion batteries are, indeed, attractive for large-scale applications.
Since the discovery of Goodenough's group, a numerous methods have been developed to synthesize LiFePO4. These preparation methods include solid-state reactions [6], mechanochemical process [7], hydrothermal approaches [8], sol-gel methods [9], co-precipitation process [10], and many more others. Through these preparation methods various LiFePO4 with different morphologies and electrochemical properties have been produced, and some of techniques have successfully been used in the industry-scale.
However, in order to utilize lithium iron phosphate as commercially viable cathode materials in lithium ion batteries, there are some hurdles need to be overcome. Pristine lithium iron phosphate compound has very poor electronic conductivity (on the order of 10−9S/cm) and slow lithium ion diffusion in solid phase. The diffusion coefficients of lithium in LiFePO4 and FePO4 are 1.8×10−14 and 2×10−16 cm2s−1, respectively. There have been tremendous efforts to improve the conductivity of lithium iron phosphate during synthesis or afterward process. These efforts include reducing lithium iron phosphate particle size [11], coating lithium iron phosphate with carbon [12], doping with cations supervalent to Li+[13], and adding metal particles (such as copper or silver) [14]. Although there are several ways to improve conductivity of LiFePO4, carbon coating has been dominant among all the methods in terms of effectiveness. Supervalent cation doping seems to be an attractive method. However, subsequent studies suggest that such an improvement in electronic conductivity is not from a true lattice doping effect but a result of carbon contamination from organic precursors and/or the formation of metallic-type conductive phases (such as Fe2P) on particle surfaces under the highly reducing conditions used [15].
In terms of improvement of Lithium ion diffusion rate in the solid phases, reducing LiFePO4 particle size has been a major approach, because reduced dimension of nanomaterials can boost efficient Lithium ion and electron transport by shortening the path length over which the Lithium ion and electron have to move. There are many ways to synthesize nanostructured LiFePO4, such as producing amorphous LiFePO4 from aqueous solutions of precursors and then obtaining nano-crystalline LiFePO4 by heating amorphous LiFePO4 in certain temperatures [16], emulsion drying synthesis of LiFePO4/Carbon composite [17], sol-gel route for LiFePO4/Carbon composite [18], and synthesis of LiFePO4 nanoparticles in supercritical water [19].
Improvement of conductivity by carbon coating is obtained at the expense of reducing active materials ratio in the electrode and tends to lower the electrode overall capacity, especially the volumetric one. Decreasing the particle size of LiFePO4 can also lower the volumetric energy density, caused by the decreasing tap density due to the high surface area of nanoparticles. Also, smaller particles need more carbon and binder to bind the particles together while forming an electrode. Therefore, controlling of carbon amount used in the electrode is critical, that is, to keep the balance between gaining enough electronic conductivity and not sacrificing the energy capacity of LiFePO4 electrodes.
In order to make LiFePO4 materials a viable electrode for lithium ion batteries, achieving a high electronic conductivity, fast Li+ diffusion rate and high tap density will be the key. People have usually paid more attention on getting high conductivity and high Li+ diffusion rate, but ignored the importance of tap density of materials, which is critical to achieve high volumetric energy density. Thus there is a need to develop a technology to produce an optimal electrode to meet all the needs of a lithium ion battery.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.